1. Field of the Invention
The present invention relates to an optical transmission system which constitutes a node or a network element of a ting network that conforms to the SONET (Synchronous Optical Network) system, and more particulary, to an optical transmission system which includes a path switch (P-SW) for selectively outputting signals coming from two directions and copes with both the STS-1 (Synchrinous Transport Signal-level 1) mode STS-3c (Synchronous Transport Signal-level 3 concatenation) mode.
2. Description of the Related Art
Due to the recent progression of the information society, there are demands for multifarious targets of information communications (i.e., multi-media of voices, images and data), a larger communication traffic capacity and a wider communications area. To meet those demands, the international standardization of the communications interface has been established mainly by. for example, ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), former CCITT (Comite Consultatif International Telegraphique et Telephonique). Proposed as one of such standard systems is the SONET system standardized in North America with the United States as the center. For example, FLM (Fiber Looped Multiplexer) 150/600/2400 ADM (Add-Drop Multiplexer) apparatuses have been designed and developed as the optical transmission system which conforms to the SONET standards.
In a ring network system which conforms to the SONET standards, an optical transmission system constituting a network element receives, for example, an STS-1 signal and an STS-3c signal, both conforming to the SONET standards, from paths in two directions (e.g., east (E) and west (W)), and selectively outputs the signal from either the eastward path or the westward path the through a path switch. Hereinafter, the eastward path and westward path will be called "E path" and "W path," respectively.
To realize the path switching operation of an optical transmission system such as the aforementioned FLM 150/600/2400 ADM equipment, it is necessary to execute the wait-to-restore (WTR) process after detection of an alarm in signals in the aforementioned two directions, count the number of path switching operations and provide an alarm display based on the result of the alarm detection. With regard to the WTR process, particularly, "SONET Add-Drop Multiplex Equipment (SONET ADM) Generic Criteria" (TR-TSY-000496, Issue 2), one of the technical references for the SONET standards, originally proposed by Bellcore (Bell Communications Research), defines the following as a requirement.
"The revertive path switching algorithm shall use a hysteresis method to prevent unwanted oscillations between service and protection paths. A provisionable wait-to-restore time of 0-12 minutes shall be provided before allowing restoration."
The service and protection paths are respectively a path of the current use and a spare path, which are assigned to the aforementioned E path and W path.
The conventional optical transmission system includes a wait-to-restore circuit for the WTR process, a P-SW (path switch) counter for counting the number of path switching operations and an alarm display circuit for displaying alarm information separately for each of the STS-1 mode and STS-3c mode.
FIG. 1 shows one example of the principle structure of the thus structured optical transmission system.